Technical Field
[0001] The present document relates to linear regulators. In particular, the present document
relates to a method and a system for increasing the open loop gain of linear regulators.
Background
[0002] Increasing the open loop gain of an amplifier is a method that may be used to improve
the performance of a linear regulator comprising the amplifier. One method for increasing
or boosting the open loop gain is the use of cascade stages. However, such methods
may introduce drawbacks as they increase the design complexity and as they may lead
to stability issues. Hence, there is a great interest at gain boosting methods which
do not add hardware overhead.
[0003] A further method for boosting the open loop gain is to use a positive feedback. However,
using a positive feedback may force the amplifier to an unstable state during operation.
[0004] Power management blocks such as linear drop-out regulators (LDO) may also take advantage
of amplifiers with gain boosters. The gain boosting in LDOs may improve the power
supply rejection ratio (PSR) and load regulation values. However, gain boosting methods
which use positive feedback are typically limited to a positive feedback gain
γ < ½. Keeping the positive feedback gain
γ < ½ typically ensures stability, however, such values limit the possibilities for
gain boosting. In other words, amplifiers in LDOs may incorporate positive feedback
for gain boosting but only with a limited gain.
[0005] The present document addresses the technical problem of providing amplifiers with
an increased open loop gain. In particular, the present document addresses the technical
problem of providing linear regulators with gain boosting and possibly with no hardware
overhead. For this purpose, a method for selecting appropriate values for the feedback
gain
γ and for selecting a pole of the feedback structure are described. By doing this,
it is possible to achieve unconditionally stable gain boosted amplifiers / regulators
with positive feedbacks.
Summary
[0006] According to an aspect, a linear regulator configured to derive an output voltage
from an input voltage is described. In particular, the linear regulator may be or
may comprise a low drop-out regulator. The linear regulator comprises an amplifier
configured to derive an amplifier output signal (at an output node of the amplifier)
from an amplifier input signal (at an input node of the amplifier). The amplifier
may comprise a differential amplifier. Furthermore, the linear regulator comprises
a pass device configured to convert the amplifier output signal (at the output node
of the amplifier, which may correspond to an input node of the pass device) into the
output voltage (at an output node of the pass device). The pass device may comprise
a metal oxide semiconductor (MOS) transistor, e.g. an N-type MOS transistor.
[0007] The linear regulator further comprises a positive feedback loop configured to determine
a positive feedback signal from the amplifier output signal, using a positive feedback
gain
γ. In particular, the positive feedback loop may be configured to determine the positive
feedback signal by multiplying the amplifier output signal with the positive feedback
gain
γ.
[0008] In addition, the linear regulator comprises a negative feedback loop configured to
determine a negative feedback signal from the output voltage, using a negative feedback
gain
β. In particular, the negative feedback loop may be configured to determine the negative
feedback signal by multiplying the output voltage with the negative feedback gain
β.
[0009] Furthermore, the linear regulator comprises a combining unit configured to determine
the amplifier input signal from the input voltage, from the positive feedback signal
and from the negative feedback voltage. In particular, the combining unit may be configured
to determine the amplifier input signal by adding the positive feedback signal to
the input voltage and by subtracting the negative feedback voltage from the input
voltage.
[0010] A transfer function of the linear regulator may exhibit a first and a second pole
at a first frequency
wp1 and at a second frequency
wp2, respectively. In other words, the linear regulator, in particular the amplifier
and/or the pass device, may be designed such that the transfer function of the linear
regulator exhibits at least two poles. The provision of at least two poles enables
the provision of a linear regulator having a high open loop gain, thereby providing
e.g. a linear regulator having a low power supply rejection ratio (PSR). At the same
time, the provision of at least two poles ensures the stability of the operation of
the linear regulator in an extended frequency range. In addition, it should be noted
that the increase open loop gain and stability can be provided without the need for
additional hardware.
[0011] The second frequency
wp2 may be greater than the first frequency
wp1. In particular, the second frequency
wp2 may be greater than the first frequency
wp1 by 3, 4, 5 or more orders of magnitude. By increasing the second frequency
wp2, the stable frequency range of the linear regulator may be extended.
[0012] The first pole may be associated with the output node of the amplifier (wherein the
output node of the amplifier carries the amplifier output signal), and the second
pole may be associated with the output node of the pass device (wherein the output
node of the pass device carries the output voltage). By way of example, the amplifier
may exhibit the first pole at the first frequency
wp1 and the pass device may exhibit the second pole at the second frequency
wp2.
[0013] As indicated above, the provision of a linear regulator having at least two poles
allows the provision of a stable linear regulator with high open loop gain. In particular,
this may be achieved by appropriately designing the positive feedback loop. In particular,
this may be achieved by selecting the positive feedback gain
γ to be 0.8 or greater, 0.9 or greater, 1.0 or greater.
[0014] As outlined above, the amplifier may comprise a differential amplifier. The differential
amplifier may comprise a differential pair comprising a first (e.g. a negative side)
input transistor and a second (e.g. a positive side) input transistor. The first and
second input transistors may be arranged in series with a first and a second load
diode, respectively. The positive feedback loop may comprise a first mirror transistor
forming a current mirror with the first load diode and a second mirror transistor
forming a current mirror with the second load diode. The first mirror transistor may
be arranged in series with the second input transistor and the second mirror transistor
may be arranged in series with the first input transistor. Hence, the positive feedback
loop may be implemented using current mirrors which provide an amplified version of
the current on one side of the differential amplifier to the respective other side
of the differential amplifier. Each of the current mirrors may provide the positive
feedback gain
γ, i.e. the current which is provided to the respective other side of the differential
amplifier may be amplified or attenuated by the value
γ.
[0015] As will be outlined in more detail in the present document, the use of a differential
amplifier provides interesting properties regarding the closed loop gain of the linear
regulator and regarding the output impedance of the linear regulator.
[0016] The input voltage may be applied to a gate of the second (positive side) input transistor.
Furthermore, the output voltage may be fed back to a gate of the first (negative side)
input transistor to provide the negative feedback loop. Hence, the linear regulator
comprising the positive and the negative feedback loop may be provided using a differential
amplifier.
[0017] According to a further aspect, a method for providing a linear regulator having a
high open loop gain is described. The linear regulator is configured to derive an
output voltage from an input voltage. The method comprises deriving an amplifier output
signal from an amplifier input signal using an amplifier. Furthermore, the method
comprises converting the amplifier output signal into the output voltage using a pass
device. In addition, the method comprises determining a positive feedback signal from
the amplifier output signal, using a positive feedback gain
γ, and determining a negative feedback signal from the output voltage, using a negative
feedback gain
β. The method comprises further determining the amplifier input signal from the input
voltage, from the positive feedback signal and from the negative feedback voltage.
The amplifier and the pass device may be selected such that a transfer function of
the linear regulator exhibits a first and a second pole at a first frequency
wp1 and at a second frequency
wp2.
[0018] It should be noted that the methods and systems including its preferred embodiments
as outlined in the present document may be used stand-alone or in combination with
the other methods and systems disclosed in this document. In addition, the features
outlined in the context of a system are also applicable to a corresponding method.
Furthermore, all aspects of the methods and systems outlined in the present document
may be arbitrarily combined. In particular, the features of the claims may be combined
with one another in an arbitrary manner. In the present document, the term "couple"
or "coupled" refers to elements being in electrical communication with each other,
whether directly connected e.g., via wires, or in some other manner.
Short description of the Figures
[0019] The invention is explained below in an exemplary manner with reference to the accompanying
drawings, wherein
Fig. 1 illustrates a block diagram of an example linear regulator;
Figs. 2 to 4 show example open loop gains for the linear regulator of Fig. 1;
Fig. 5 shows a block diagram of an example linear regulator;
Fig. 6 shows a circuit diagram of an example amplifier with positive feedback;
Figs. 7, 8a and 8b show example open loop gains for the linear regulator of Fig. 5;
Fig. 9 shows a flow chart of an example method for providing a linear regulator with
high open loop gains; and
Fig. 10 shows example open loop and closed loop transfer functions of the amplifier
of Fig. 6.
Detailed Description
[0020] As outlined in the introductory section, the present document addresses the technical
problem of providing stable amplifiers with an increased open loop gain. In particular,
a method is described which allows achieving theoretically infinite open loop gains
with a single error amplifier stage. As a result of this, the performance of LDOs
may be improved with no hardware overhead. In particular, the PSR and load regulation
and other performance metrics associated with the open loop gain of an amplifier may
be improved.
[0021] The proposed methods allow the available hardware to be used more efficiently. Furthermore,
the specifications of the amplifiers may be relaxed. In addition, the stability of
LDOs which use positive feedback in the amplifiers may be improved. The proposed method
may be used in various different types of LDOs since a positive feedback loop can
be part of the amplifier structure in the form of dynamic biasing or similar. Furthermore
by boosting the positive feedback of the load to values larger unity, negative output
impedance of the linear regulator may be achieved.
[0022] Fig. 1 shows a block diagram of an example LDO 100. The LDO 100 comprises one or
more amplification stages 102 and a pass device 103. Furthermore, the LDO 100 comprises
a positive feedback 104 with a positive feedback gain γ. Furthermore, the LDO 100
comprises a negative feedback 105 with a negative feedback gain
β. The positive feedback and the negative feedback are combined with an input voltage
111 of the LDO 100 using the combining unit 101. Hence, the input signal to the one
or more amplification stages 102 (referred to in the following as the amplifier 102)
is the sum of the input voltage 111 and the positive feedback signal minus the negative
feedback signal. The LDO 100 may be configured to derive an output voltage 112 from
the input voltage 111 for a load 106 of the linear regulator 100.
[0023] In other words, the LDO 100 of Fig. 1 has two feedback loops, a first feedback loop
with positive feedback and a second feedback loop with negative feedback. In case
of a linear regulator circuit the pass device 103 is positioned between the two feedback
loops. The positive feedback with the positive feedback gain γ may be embedded inside
the amplifier 102. The value of the positive feedback gain γ and the placement of
the poles of the LDO 100 may be selected to provide a stable LDO 100 with a high open
loop gain.
[0024] The transfer function of the LDO 100 may be written as:
where Av is the gain of the amplifier 102, where Ap is the gain of the pass device
103, where s is the (complex) frequency, where wp1 is a first pole of the transfer
function at the output node 107 of the amplifier 102, and where wp2 is a second pole
of the transfer function at the output node 108 of the pass device 103. The effect
of positive and negative feedback can be seen from the above formula. At DC (i.e.
at s = 0), the transfer function simplifies to
[0025] In the frequency region where s « wp2, the term
and the transfer function becomes
[0026] The above formula is applicable in particular for wp1 < wp2, and possibly s >
wp1 or s >> wp1.
[0027] For the case s » wp2 and s » wp1, the transfer function becomes
[0028] The above formulas may be used to analyze the LDO 100 of Fig. 1. In particular, the
open loop gain (with the negative feedback loop being interrupted, i.e. with the negative
feedback gain
β = 0) may be analyzed as a function of frequency s. In this case, we have for wp1
<< wp2:
[0029] Fig. 2 illustrates the magnitude of the open loop gain (reference numeral 201), i.e.
in dB for different frequencies s and for different values of the positive feedback
gain
γ, i.e.
γ = 0.8 and
γ = 1.2. Furthermore, Fig. 2 illustrates the phase of the open loop gain (reference
numeral 202) for different frequencies s and for different values of the positive
feedback gain
γ, i.e.
γ = 0.8 and
γ = 1.2. The two poles
wp1 (reference numeral 203) and wp2 (reference numeral 204) can be observed.
[0030] Fig. 3 illustrates the magnitude of the open loop gain (reference numeral 201), i.e.
in dB for different frequencies s and for a positive feedback gain
γ = 1 . Furthermore, Fig. 3 illustrates the phase of the open loop gain (reference
numeral 202) for different frequencies s and for a positive feedback gain
γ = 1.
[0031] Overall, it can be observed that by providing a second pole wp2 which is greater
than the first pole
wp1, the phase of the open loop gain can be maintained in-phase, i.e. between 0 and
180 degrees, even for a positive feedback gain
γ = 1 or greater than one. Furthermore, it can be observed that by selecting a positive
feedback gain
γ = 1, substantial values for the magnitude of the open loop gain can be achieved at
low frequencies (in particular for s = 0). The value of the magnitude of the open
loop gain at zero frequency typically impacts the PSR. In particular, a high open
loop gain at zero frequency typically leads to a high PSR.
[0032] Various techniques may be used to place the first pole wp1 to have a lower frequency
value than the second pole wp2. The Miller effect can be used with the gain of the
error amplifier 102 to boost an internal capacitor connected to node Vm 107 of the
linear regulator 100 shown in Fig. 1. Also the gate parasitic (capacitance) associated
with the pass device 103 may be added to the total capacitor at node Vm 107, which
will further increase the total capacitor.
[0033] Fig. 4 illustrates the magnitude of the open loop gain 201 and the phase 202 at zero
frequency for different positive feedback gains
γ 401 ranging from 0.8 to 1.2. It can be seen that the magnitude of the open loop gain
201 peaks at
γ = 1, thereby allow for high PSRs.
[0034] In other words, Fig. 3 depicts the case where
γ = 1 and Figs. 2 and 4 show the magnitude 201 and response 202 of the linear regulator
100 shown in Fig. 1 for positive feedback gain values varying from
γ = 0.8 to
γ = 1.2. From Figure 2 two poles 203, 204 can be observed. Fig. 4 provides information
regarding the phase changes at low frequencies (s = 0) depending on the value of
γ. For the low values of
γ, a180 degree phase can be observed. As the value of the positive feedback gain
γ is increased above one, in-phase operation for low frequencies can be observed. Hence,
the proper pole placement allows for the design of a linear regulator 100 which is
stable and which takes advantage of high open loop gains.
[0035] In the following, the output impedance of a closed loop linear regulator 100 is analyzed,
in particular for the case where the positive feedback gain is
γ ≥ 1.
[0036] In an example, an LDO comprises an amplifier 502 with a positive feedback loop as
gain booster. Fig. 5 illustrates a basic LDO structure with an NMOS (N-type metaloxide
semiconductor) pass device 103. It should be noted that LDOs with a PMOS pass device
may be used as well. The output 112 is fed back to the amplifier 502 using a resistor
505 to form a negative feedback loop.
[0037] Fig. 6 shows an example circuit used for the amplifier 502. The positive feedback
103 is formed via cross coupled connection. This embodiment is a preferred embodiment
in which the positive feedback is embedded within the amplifier structure 502. Various
types of positive feedbacks may be used.
[0038] As outlined above, the use of positive feedback gains of
γ ≥ 1 is enabled by an appropriate placement of the poles wp1 and wp2. The poles are
associated with the output node of the amplifier 102, 502 and with the output node
of the pass device 103, respectively. As shown in the present document, LDOs with
a high open loop gain may be provided by providing a second pole wp2 at high frequencies.
[0039] It should be noted that in case of a load current
Iout with a fixed reference at the positive input in normal operation, the feedback voltage
Vip =
V- =
Vout on the negative input is usually smaller than the input voltage
Vin (when using only negative feedback). This difference is setting the output current
Iout equal to the load current. This is caused by the negative feedback behaviour. The
negative change in output voltage is causing a larger output (positive) current to
counteract and vice versa. An equivalent circuit would be a voltage source with a
resistor in series. In case of a load current the voltage across the load becomes
smaller.
[0040] Fig. 6 also shows example currents flowing within the amplifier 502 comprising the
positive feedback. The negative feedback may be provided by feeding back the output
voltage
Vout 112 to the negative input
Vip of the differential pair. The left side (negative side) diode 601 and the right side
(positive side) diode 602 are traversed by the currents
IL and
IR, respectively. The positive feedback 104 is provided by feeding back the currents
through the diodes 601, 602 to the respective other branch of the differential amplifier
using current mirrors. For this purpose, the amplifier 502 comprises a left side (negative
side) transistor 611 which forms a current mirror with the left side diode 601. The
current through the left side transistor 611 is
γ·IL, wherein
γ is the positive feedback gain. The current
γ·IL is coupled to the right side (positive side) branch of the differential pair. Furthermore,
the amplifier 502 comprises a right side (positive side) transistor 612 which forms
a current mirror with the right side diode 602. The current through the right side
transistor 612 is
γ·IR, wherein
γ is the positive feedback gain. The current
γ·IR is coupled to the left side (negative side) branch of the differential pair.
[0041] From Fig. 6 it can be seen that the output or load current
Iout is
Iout = M·IR - M·IL, wherein M is the gain of the current mirror formed by the right side diode 602 and
the right side output transistor 622, and wherein M is the gain of the current mirror
formed by the left side diode 601 and the left side output transistors 621, 623, 624.
Furthermore, it can be seen that the current
IB -
ΔI through the right side input transistor 604 is
IB - ΔI=
IR +
γ·IL; and that the current
IB +
ΔI through the left side input transistor 603 is
IB +
ΔI=
IL +
γ·IR, wherein 2·
IB is the bias current provided by a current source 605 of the amplifier 502.
[0042] The above equations provide: -2·
ΔI =
IR - IL +
γ·(IL - IR), and with
one obtains:
[0043] The differential voltage
ΔV at the input of the amplifier 502 is given by the difference between the input voltage
Vin and the output voltage
Vout, which is fed back to the negative input of the amplifier 502,
ΔV =
Vin - Vout. The current difference
ΔI may also be written as
wherein
gm is the transconductance
gm of the amplifier 502 without the positive feedback. By considering
and
the load current
Iout becomes
Iout =
geff·
ΔV, wherein
geff is the effective transconductance of the amplifier 502 including the positive feedback
with
[0044] From the above formula, it can be seen that for
γ > 1, the effective transconductance, and by consequence also the output impedance,
of the amplifier 502 becomes negative. This leads to the effect that in case of
γ > 1, the output voltage
Vout increases in case of an increasing load current
Iout.
[0045] In other words, if a boosted (γ>1) and stable version of the amplifier 502 is used,
the above equations indicate that in order to achieve an equilibrium a higher voltage
on the negative input is obtained. Again the output current
Iout is increasing when the output voltage
Vout drops, which is due to the negative feedback behaviour, however, the steady state
is reached, when the negative side voltage is higher than the positive reference voltage
Vin to equalize the load current
Iout. An equivalent circuit to the amplifier 502 would make use of a negative resistor
to represent this behaviour with a minimal number of components.
[0046] The open loop transfer function shows a hysteretic behaviour similar to a Schmitt
trigger with a typical meta-stable area. However the overall negative feedback is
linearizing this into a monotonic transfer function. This is illustrated in Fig. 10
which shows the open loop transfer function of the amplifier 502 for different values
of the positive feedback gain
γ. The open loop transfer function is given by the ratio
Vout/
ΔVin for the case where the negative feedback loop is open. It can be seen that the transfer
functions 911, 912, 913 exhibit an increasing gradient from
γ = 0 up to
γ = 1. At
γ = 1 the gain is at infinity. This increasing open loop gain is also shown in Fig.
4. For
γ > 1 the open loop transfer function 914 exhibits a hysteretic behaviour.
[0047] Fig. 10 also shows the closed loop transfer functions
Vout/
ΔV for the amplifier 502 having a closed negative feedback loop. It can be seen that
for
γ < 1 and for
γ > 1 the closed loop transfer functions 921, 923 exhibit a positive gain. Furthermore,
Fig. 10 shows the output impedances
Vout/
Iout for
γ < 1 and for
γ > 1. It can be seen that for
γ < 1 the output impedance 922 is positive and for
γ > 1 the output impedance 924 is negative. An amplifier 502 having a negative output
impedance 924 may be beneficial for providing regulators 100 with a reduced overall
output impedance. Alternatively or in addition, such an amplifier 502 may be used
to forward compensate a voltage drop caused by a loading current without having a
feedback loop.
[0048] Fig. 7 shows the magnitude of the open loop gain 201 and the phase of the open loop
gain 202 for the LDO shown in Figs. 5 and 6. The magnitude 201 and phase 202 are shown
for a positive feedback gain of
γ = 1. It can be seen that high (magnitudes of the) open loop gain 201 may be achieved
for low frequencies. As a result of this, high PSR may be achieved with the LDO shown
in Figs. 5 and 6.
[0049] Fig. 8a shows the magnitude of the open loop gain 201 and Fig. 8b shows the phase
of the open loop gain 202 for positive feedback gains ranging from
γ = 0.8 to
γ = 1.2. From Fig. 8b, it can be seen that for
γ > 1, the phase 202 starts at 0 degrees for low frequencies. Hence, selecting higher
values of
γ ensures more stable frequency regions considering the phase inversion point. Consequently,
the LDO shown in Figs. 5 and 6 allows achieving a higher loop gain and providing an
increased stable operation region. Transient simulations for
γ = 1.2 have shown that even for such extreme values of the positive feedback gain
γ the LDO operation remains stable.
[0050] Fig. 9 shows a flow chart of an example method 900 for providing a linear regulator
100 having a high open loop gain. The linear regulator 100 is configured to derive
an output voltage 112 from an input voltage 111. The method 900 comprises deriving
901 an amplifier output signal from an amplifier input signal using an amplifier 102,
e.g. a differential amplifier. The method 900 further comprises converting 902 the
amplifier output signal into the output voltage 112 using a pass device 103. In addition,
the method 900 comprises determining 903 a positive feedback signal from the amplifier
output signal, using a positive feedback gain
γ 104, and determining 904 a negative feedback signal from the output voltage 108,
using a negative feedback gain
β 105. The amplifier input signal is determined (step 905) from the input voltage 111,
from the positive feedback signal and from the negative feedback voltage. In addition,
the method 900 comprises selecting 906 the amplifier 102 and the pass device 103 such
that a transfer function of the linear regulator 100 exhibits a first and a second
pole at a first frequency
wp1 and at a second frequency
wp2. Typically, the poles are designed such that the second frequency
wp2 is substantially higher than the first frequency
wp1 (e.g. by several orders of magnitude).
[0051] In the present document, a linear regulator structure has been described which allows
achieving a high open loop gain and stable operation.
[0052] It should be noted that the description and drawings merely illustrate the principles
of the proposed methods and systems. Those skilled in the art will be able to implement
various arrangements that, although not explicitly described or shown herein, embody
the principles of the invention and are included within its spirit and scope. Furthermore,
all examples and embodiment outlined in the present document are principally intended
expressly to be only for explanatory purposes to help the reader in understanding
the principles of the proposed methods and systems. Furthermore, all statements herein
providing principles, aspects, and embodiments of the invention, as well as specific
examples thereof, are intended to encompass equivalents thereof.
1. A linear regulator (100) configured to derive an output voltage (112) from an input
voltage (111), the linear regulator (100) comprising,
- an amplifier (102) configured to derive an amplifier output signal from an amplifier
input signal;
- a pass device (103) configured to convert the amplifier output signal into the output
voltage (112);
- a positive feedback loop configured to determine a positive feedback signal from
the amplifier output signal, using a positive feedback gain γ (104);
- a negative feedback loop configured to determine a negative feedback signal from
the output voltage (108), using a negative feedback gain β (105); and
- a combining unit (101) configured to determine the amplifier input signal from the
input voltage (111), from the positive feedback signal and from the negative feedback
voltage; wherein a transfer function of the linear regulator exhibits a first and
a second pole at a first frequency wp1 and at a second frequency wp2, respectively.
2. The linear regulator (100) of claim 1, wherein the second frequency wp2 is greater than the first frequency wp1.
3. The linear regulator (100) of claim, wherein the second frequency wp2 is greater than the first frequency wp1 by 3, 4, 5 or more orders of magnitude.
4. The linear regulator (100) of any previous claim, wherein
- the first pole is associated with an output node of the amplifier (102); and
- the second pole is associated with an output node of the pass device (103).
5. The linear regulator (100) of claim 4, wherein
- the amplifier (102) exhibits the first pole at the first frequency wp1; and
- the pass device (103) exhibits the second pole at the second frequency wp2.
6. The linear regulator (100) of any previous claim, wherein the positive feedback gain
γ is 0.8 or greater, 0.9 or greater, 1.0 or greater.
7. The linear regulator (100) of any previous claim, wherein the amplifier (102) comprises
a differential amplifier.
8. The linear regulator (100) of claim 7, wherein
- the differential amplifier comprises a differential pair (603, 604) comprising a
first input transistor (603) and a second input transistor (604);
- the first and second input transistors (603, 604) are arranged in series with a
first and a second load diode (601, 602), respectively;
- the positive feedback loop comprises a first mirror transistor (611) forming a current
mirror with the first load diode (601) and a second mirror transistor (612) forming
a current mirror with the second load diode (602);
- the first mirror transistor (611) is arranged in series with the second input transistor
(604); and
- the second mirror transistor (612) is arranged in series with the first input transistor
(603).
9. The linear regulator (100) of claim 8, wherein the current mirrors provide the
positive feedback gain γ (104).
10. The linear regulator (100) of any of claims 8 to 9, wherein
- the input voltage (111) is applied to a gate of the second input transistor (604);
and
- the output voltage (112) is fed back to a gate of the first input transistor (603)
to provide the negative feedback loop.
11. The linear regulator (100) of any previous claim, wherein the pass device (103) comprises
a metal oxide semiconductor transistor.
12. The linear regulator (100) of any previous claim, wherein the combining unit (101)
is configured to determine the amplifier input signal by adding the positive feedback
signal to the input voltage (111) and by subtracting the negative feedback voltage
from the input voltage (111).
13. The linear regulator (100) of any previous claim, wherein the positive feedback loop
is configured to determine the positive feedback signal by multiplying the amplifier
output signal with the positive feedback gain γ (104).
14. The linear regulator (100) of any previous claim, wherein the negative feedback loop
is configured to determine the negative feedback signal by multiplying the output
voltage (108) with the negative feedback gain β (105).
15. A method (900) for providing a linear regulator (100) having a high open loop gain,
wherein the linear regulator (100) is configured to derive an output voltage (112)
from an input voltage (111); the method (900) comprising
- deriving (901) an amplifier output signal from an amplifier input signal using an
amplifier (102);
- converting (902) the amplifier output signal into the output voltage (112) using
a pass device (103);
- determining (903) a positive feedback signal from the amplifier output signal, using
a positive feedback gain γ (104);
- determining (904) a negative feedback signal from the output voltage (108), using
a negative feedback gain β (105);
- determining (905) the amplifier input signal from the input voltage (111), from
the positive feedback signal and from the negative feedback voltage; and
- selecting (906) the amplifier (102) and the pass device (103) such that a transfer
function of the linear regulator (100) exhibits a first and a second pole at a first
frequency wp1 and at a second frequency wp2, respectively.